Not that MythBusters is uberly scientific but:
So basically, as long as you don't sit there and try to burn out your fish's eyes I'd say it's safe enough.
ETA: jwebbe beat me to it!
Someone in the bay area got busted yesterday for pointing a laser pointer at a helicopter and a cop - they pinpointed ( no pun intended ) to the exact location and got the guy. Although I am sure he probably had a high powered version of the regular laser pointer... Personally, I use mine to keep the pigeons OFF OF my neighbors roofline - they come there and destroy the wood, etc - seems to keep them off of the roof for a few days and then they come back